Power modulated endometrial lining tissue ablation

ABSTRACT

A system is configured to delivering radiofrequency power to the endometrial lining tissue of a uterine cavity, including modulating the delivered power so that a measured impedance of the endometrial lining tissue tracks a target impedance as a function of time, wherein the target tissue impedance is derived from a function that approximates a preferred endometrial lining tissue ablation impedance curve that is determined based upon a measured impedance of the endometrial lining tissue after RF power has been delivered for a predetermined initial time period.

RELATED APPLICATION DATA

The present application is a continuation of U.S. patent applicationSer. No. 16/001,366, filed Jun. 6, 2018, which is a continuation of U.S.Pat. No. 10,004,553, filed Dec. 17, 2014, which claims the benefit under35 U.S.C. § 119 to U.S. provisional patent application Ser. No.61/920,152, filed Dec. 23, 2013. The foregoing applications are eachhereby incorporated by reference into the present application in theirentirety.

FIELD OF INVENTION

The disclosed inventions pertain generally to systems and methods forthermally treating the interior surfaces of body organs. Morespecifically, the disclosed inventions pertain to a system and methodfor ablating the endometrial lining tissue of the uterus.

BACKGROUND

Thermal ablation or coagulation of the interior lining of a body organis a procedure which involves heating the organ lining to a temperaturethat destroys the cells of the lining tissue. Such a procedure may beperformed as a treatment to one of many conditions, such as menorrhagia,which is characterizes by chronic bleeding of the endometrial tissuelayer of the uterus. Existing methods for effecting thermal ablation ofthe endometrial lining tissue include circulation of heated fluid insidethe uterus (either directly or inside a balloon placed in the uterus),laser treatment of the lining, and resistive heating using applicationof RF energy to the tissue to be ablated. Techniques using RF energyprovide an RF electrical signal to one or more electrodes in contactwith the subject organ tissue. Electrical current flows from theelectrodes and into the organ tissue. The current flow resistively heatsthe surrounding tissue. Eventually, the heating process destroys thecells surrounding the electrodes and thereby effectuates ablation.

For example, U.S. Pat. No. 5,769,880 (Truckai et al.) and U.S. Pat. No.6,508,815 (Strul et al.) describe a system for endometrial lining tissueablation using an electrode carrying member to transmit radiofrequency(RF) energy to cause thermal heating (and, thus ablation) of the tissue,wherein the electrode carrying member is substantially absorbent orpermeable to moisture and gases such as steam, and is conformable to theuterine cavity. Suctioning means may additionally be positioned withinthe electrode carrying member to aid in the removal of moisture, whethergas or liquid, present or generated during the ablation procedure. Anarray of electrodes is mounted to the surface of the electrode carryingmember, and arranged to produce ablation to a predetermined depth. Theelectrodes may be provided with means for variably controlling ablationdepth by changing the electrode density or center to center spacing.Following placement of the ablation device within the patient's uterus,so that the electrodes are in contact with the tissue to be ablated, anRF generator is used to deliver RF energy to the electrodes and tothereby induce current flow from the electrodes to tissue to be ablated.As the current heats the tissue, moisture (such as steam or liquid)leaves the tissue causing the tissue to dehydrate. The moisturepermeability or absorbency of the electrode carrying member allows formoisture to leave the ablation site so as to prevent the moisture fromproviding a path of conductivity for the current. The systems, devicesand methods disclosed and described in U.S. Pat. Nos. 5,769,880 and6,508,815 are well-suited for performing endometrial tissue ablationprocedures, e.g., for treating Menorrhagia, the medical term forexcessively heavy menstrual bleeding, and are embodied in the NovaSure®endometrial ablation system manufactured and distributed by Hologic,Inc., based in Bedford, Mass. U.S. Pat. Nos. 5,769,880 and 6,508,815 areboth hereby fully incorporated by reference.

However, use of the NovaSure endometrial ablation system requires theoperator to first measure the length and width of the uterine cavity,and input these dimensions into the controller in order to establish thepower level for conducting the endometrial lining ablation procedure.This is because the amount of endometrial lining tissue to be ablated isproportional to the size of the uterine cavity. Additionally, proceduretime can vary from as short as approximately one minute in duration, toas long as two minutes in duration, depending on a number of factorse.g., the amount of moisture in the tissue, in addition to the uterinecavity size.

In particular, any RF tissue ablation system must accurately determinethe appropriate level of applied power for and during a procedure. Thispower level must provide sufficient heating to effectuate a complete,(i.e., uniform thickness) ablation. At the same time, however, the powerlevel must be controlled to prevent over-ablation. Moreover, an RFgenerator must be controlled to respond dynamically to changes in theimpedance of the subject tissue.

U.S. Pat. No. 5,954,717 (Behl, et al.) discloses and describes a systemand methods for heating solid body tissue (e.g., a tumor) by deliveringradio frequency energy through tissue electrodes, wherein the deliveredenergy is initially controlled to cause an abrupt increase in impedancebetween the electrodes and the tissue, observed in the form of an abruptdecrease in power delivered to the electrodes. The power at which theimpedance increases or the time required to induce such an increase inimpedance, are relied upon to determine a desired procedure power levelfor achieving a maximum sustainable delivery of radio frequency energyto the tissue to achieve complete and uniform heating of the solidtissue volume.

U.S. Pat. No. 6,033,399 (Gines) discloses and describes anelectrosurgical generator having an output power control system thatemploys tissue impedance feedback to cause the impedance of vesseltissue to rise and fall in a cyclic pattern until the tissue isdesiccated. The stated advantage of the disclosed power control systemis that thermal spread (i.e., non-uniform heating) and tissue-charring(i.e., overheating) are reduced. The output power is applied cyclicallyby a control system with tissue impedance feedback. The impedance of thetissue follows the cyclic pattern of the output power several times,depending on the state of the tissue, until the tissue becomes fullydesiccated. High power is applied to cause the tissue to reach a highimpedance, and then the power is reduced to allow the impedance to fall,whereby the delivered thermal energy dissipates during the low powercycle. The control system is said to be adaptive to the tissue in thesense that output power is modulated in response to the impedance of thetissue.

U.S. Pat. No. 6,843,789 (Goble discloses and describes an apparatus foruse in performing ablation of organs (such as the uterus), and othertissues, including a radio frequency generator which provides a radiofrequency signal to ablation electrodes. The power level of the radiofrequency signal is determined based on the subject area of ablation,and is coupled with the ablation electrodes through a transformationcircuit. The transformation circuit includes a high impedancetransformation circuit, and a low impedance transformation circuit. Thehigh or low impedance transformation circuit is selected based on theimpedance of the ablation electrodes in contact with the subject tissue.Vacuum level, impedance level, resistance level, and time are measuredduring ablation. If these parameters exceed determinable limits theablation procedure is terminated.

U.S. Pat. No. 6,843,789 (Goble) discloses and describes anelectrosurgical system, including an electrosurgical generator and abipolar electrosurgical instrument, the generator being configured(i.e., programmed) to perform a tissue treatment cycle in which radiofrequency energy is delivered to the electrosurgical instrument as anamplitude-modulated radio frequency power signal in the form of asuccession of pulses characterized by successive pulses of progressivelyincreasing pulse width and progressively decreasing pulse amplitude.There are periods of at least 100 milliseconds between successivepulses, and the treatment cycle begins with a predetermined pulsemark-to-space ratio. Energy delivery between pulses is substantiallyzero. Each burst is of sufficiently high power to form vapor bubbleswithin tissue being treated and the time between successive pulses issufficiently long to permit condensation of the vapor.

U.S. Pat. No. 8,152,801 (Goldberg, et al.) discloses and describes solidtissue ablation systems and methods, in which radio frequency energy isdelivered to the tissue, and a physiological parameter (e.g., impedanceor temperature) indicative of a change in moisture concentration of thetissue is sensed. The ablation energy is alternately pulsed on and offto generate an energy pulse train, with the ablation energy being pulsedON if the sensed physiological parameter crosses a threshold valueindicative of an increase in the moisture concentration, and pulsed OFFif the sensed physiological parameter crosses a threshold valueindicative of a decrease in the moisture concentration.

U.S. Pat. No. 8,241,275 (Hong et al.) discloses and describes a methodof applying ablation energy for transmural tissue wall ablation, e.g.,for treating afibrillation, including applying ablation energy at astarting power to a tissue site and monitoring the impedance of thetissue site. Thereafter, the power applied to the tissue site is reducedas a function of a rate of an increase in tissue impedance.

However, none of the foregoing references addresses the specificproblems encountered in endometrial lining tissue ablation procedures,for example, accounting for differences in uterine size, or otherwiseproviding for a uniform and complete endometrial lining ablation in asubstantially uniform time period, regardless of differences in uterinecavity size, tissue moisture, or other factors.

SUMMARY

The disclosed embodiments are directed to systems and methods forachieving clinically successful endometrial lining tissue ablations bymodulating the delivered power during the procedure in a manner thatreaches approximately fifty ohms endometrial lining tissue impedance inapproximately two minutes, regardless of the patient's uterine cavitysize or other factors, e.g., the patient's monthly cycle, that until nowhave influenced the procedure power and duration.

In one embodiment, a method of performing an endometrial lining tissueablation procedure comprises the steps or acts of deliveringradiofrequency (RF) power to endometrial lining tissue of a uterinecavity, and modulating the delivered power so that a measured impedanceof the endometrial lining tissue tracks a target tissue impedance as afunction of time. The target tissue impedance may be derived from afunction that approximates a preferred endometrial lining tissueablation impedance curve, which is based upon a measured impedance ofthe endometrial lining tissue after RF power has been delivered for apredetermined initial time period, for example, based upon measuredimpedance data of prior performed ablation procedures. It is preferredthat the delivered RF power is substantially constant for the initialtime period, and that the initial time period and delivered RF powerlevel are together sufficient such that the measured impedance isrepresentative of the endometrial lining tissue. RF power is preferablythereafter delivered to the endometrial lining tissue until (i) ameasured impedance of the endometrial lining tissue reachespredetermined maximum tissue impedance, or (ii) a maximum ablationprocedure time is reached, whichever occurs first.

By way of non-limiting example, the preferred endometrial lining tissueablation impedance curve is based upon an ablation procedure time ofapproximately 120 seconds and a maximum tissue impedance ofapproximately 50 ohms, and wherein the function approximating thepreferred endometrial lining tissue ablation impedance curve isI_(T)=4+(49^((T/120))), where I_(T) is a target tissue impedance (inohms) for a given time T (in seconds) during the ablation time period.

Modulating the delivered RF power to the endometrial lining tissueincludes one of increasing, decreasing, or maintaining constant adelivered RF current at regular power adjustment intervals, wherein ateach power adjustment interval, the amperage of the delivered current isnot increased or decreased more than a predetermined adjustment limit.By way of non-limiting examples, the predetermined adjustment limit maybe in a range of about 2% to about 6%. In one embodiment, thepredetermined adjustment limit is 3%. In another embodiment, thepredetermined adjustment limit is 5%.

In one embodiment, a method of endometrial lining tissue ablationincludes positioning a radiofrequency (RF) applicator within a uterinecavity; delivering an RF current through the applicator to therebydeliver a corresponding RF power to endometrial lining tissue of theuterine cavity; measuring an impedance of the endometrial lining tissueafter delivery of the RF power for a predetermined initial time period;and modulating the delivered RF power so that impedance of theendometrial lining tissue tracks a target tissue impedance derived froma function that approximates a preferred endometrial lining tissueablation impedance curve, based upon the measured impedance of theendometrial lining tissue after RF power has been delivered for theinitial time period, wherein RF power is delivered to the endometriallining tissue until (i) a measured impedance of the endometrial liningtissue reaches a predetermined maximum tissue impedance, or (ii) apredetermined maximum ablation procedure time is reached, whicheveroccurs first.

In such embodiment, the target tissue impedance I_(t) for a given time tfollowing the initial time period may be calculated based upon theformula I_(t)=I_(max)−((I_(max)−I_(o))*S), where

I_(max) is a target maximum impedance,

I_(o) is a tissue impedance at time t derived from the functionapproximating the preferred endometrial lining tissue ablation impedancecurve, and

S is a scaling factor equal to (I_(max)−I_(meas))/(I_(max)−I_(io)),where I_(meas) is the measured impedance of the endometrial liningtissue after RF power has been delivered for the initial time periodt_(initial), and I_(io) is a tissue impedance at time t_(initial)derived from the function approximating the preferred endometrial liningtissue ablation impedance curve.

In still another embodiment, an endometrial tissue ablation systemincludes an RF generator operatively coupled to a controller, whereinthe controller is configured to (a) cause the generator to deliver RFpower to endometrial lining tissue of a uterine cavity electricallycoupled to the generator, and (b) modulate the delivered RF power sothat a measured impedance of the endometrial lining tissue tracks atarget tissue impedance as a function of time. By way of example, the RFpower may be delivered via an applicator including an electrode carriercomprising one or more electrodes thereon configured for beingpositioned within the uterine cavity for contacting and ablatingendometrial lining tissue of the uterine cavity.

The target tissue impedance is preferably derived from a function thatapproximates a preferred endometrial lining tissue ablation impedancecurve, scaled based upon a measured impedance of the endometrial liningtissue after RF power has been delivered for a predetermined initialtime period. The controller is preferably configured to cause the RFpower to be delivered to the endometrial lining tissue until (i) ameasured impedance of the endometrial lining tissue reachespredetermined maximum tissue impedance, or (ii) a maximum ablationprocedure time is reached, whichever occurs first.

These and further embodiments and aspects of the disclosed inventionsare described herein in detail, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation view of an exemplary prior art RF generatorand an ablation device suitable for use with the RF generator, asillustrated and described in above-incorporated U.S. Pat. No. 6,508,815.

FIG. 1B is a top view of the distal end of the ablation device of FIG.1A, shown with an applicator head in an extended position.

FIG. 1C is a perspective view of the applicator head in the extendedposition of FIG. 1B.

FIG. 1D is a front elevation view of the applicator head in the extendedposition of FIG. 1B.

FIG. 2 is a graph illustrating a model target tissue impedance curve foran endometrial tissue lining ablation procedure, assuming an initialtissue impedance of five ohms.

FIG. 3 is a table of discrete target tissue impedance values for anendometrial lining tissue ablation procedure, derived from amathematical function that approximates the model impedance curve ofFIG. 2.

FIG. 4 is a graph illustrating a scaled model target tissue impedancecurve for an endometrial tissue lining ablation procedure, assuming aninitial tissue impedance of ten ohms.

FIG. 5 is a table of discrete tissue impedance values for an endometriallining tissue ablation procedure derived from a mathematical functionthat approximates the impedance curve of FIG. 4.

FIG. 6 is a graph illustrating a scaled model target tissue impedancecurve for an endometrial tissue lining ablation procedure, assuming aninitial tissue impedance of two ohms.

FIG. 7 is a table of discrete tissue impedance values for an endometriallining tissue ablation procedure derived from a mathematical functionthat approximates the impedance curve of FIG. 6.

FIG. 8 is a compilation of the tissue impedance curves of FIG. 2, FIG.4, and FIG. 6, illustrating how each of the curves converges toapproximately fifty ohms at around the two minute mark.

DETAILED DESCRIPTION

Various embodiments of the disclosed inventions are describedhereinafter with reference to the figures. The figures are notnecessarily drawn to scale, the relative scale of select elements mayhave been exaggerated for clarity, and elements of similar structures orfunctions are represented by like reference numerals throughout thefigures. It should also be understood that the figures are only intendedto facilitate the description of the embodiments, and are not intendedas an exhaustive description of, or as a limitation on the scope of, thedisclosed inventions, which are defined only by the appended claims andtheir equivalents. In addition, an illustrated embodiment of thedisclosed inventions needs not have all the aspects or advantages shown,and an aspect or an advantage described in conjunction with a particularembodiment of the disclosed inventions is not necessarily limited tothat embodiment and can be practiced in any other embodiments even ifnot so illustrated.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” include plural referents unless the contentclearly dictates otherwise. As used in this specification and theappended claims, the term “or” is generally employed in its senseincluding “and/or” unless the content clearly dictates otherwise. Allnumeric values are herein assumed to be modified by the term “about,”whether or not explicitly indicated. The terms “about” and“approximately” generally refer to a range of numbers that one of skillin the art would consider equivalent to the recited value (i.e., havingthe same function or result). In many instances, the terms “about” and“approximately” may include numbers that are rounded to the nearestsignificant figure. As used in this specification and the appendedclaims, numerical ranges include both endpoints and all numbers includedwithin the range. For example, a range of 1 to 5 inches includes,without limitation, 1, 1.5, 2, 2.75, 3, 3.80, 4 and 5 inches.

In accordance with a general aspect of the disclosed inventions, it hasbeen discovered by the present inventors that the duration of anendometrial lining ablation procedure can be tightly controlled bymodulating the delivered power during the ablation process. Morespecifically, the present inventors have reviewed clinical data obtainedfrom a number of endometrial lining ablation procedures, and havedetermined that a clinically desirable and successful procedure may beachieved by modulating the delivered energy during the course of theprocedure in order to reach a target endometrial tissue impedance ofapproximately fifty ohms over a procedure time period lastingapproximately two minutes, regardless of the initial endometrial liningtissue impedance. In particular, if the target tissue impedance isreached in less time, there is a risk that too much energy was deliveredtoo quickly, resulting in possible tissue charring or non-uniformheating of the endometrial lining tissue, both clinically undesirableoutcomes. On the other hand, if the target tissue impedance is notreached by the end of a two minute time period, the procedure is takingtoo long, which is also clinically undesirable.

Thus, it is an object of the disclosed and described embodiments toprovide endometrial lining tissue ablation systems and methods of usingsame for achieving a clinically successful endometrial lining tissueablation by modulating the delivered power in a manner that reaches auniform target tissue impedance (e.g., approximately fifty ohms) over auniform procedure time (e.g., approximately two minutes), regardless ofuterine cavity size or other factors (e.g., the patient's monthly cycle)that can influence the amount of power that needs to be delivered for acomplete and uniform ablation.

Advantageously, such systems and methods do not require anypre-procedure measuring or input of the uterine cavity size, and allowfor a predictable and repeatable ablation procedure time regardless ofthe uterine cavity size, while maintaining uniform and complete tissueablation depths.

Exemplary Endometrial Tissue Ablation System

Advantageously, embodiments of the disclosed systems and methods may besoftware-implemented in an existing or modified controller of thecurrent NovaSure endometrial ablation system, or in a comparableendometrial lining tissue ablation system. As such, to facilitate thisdetailed description, the NovaSure endometrial ablation system willfirst be summarily described, with further reference also made to theabove-incorporated U.S. Pat. No. 6,508,815. Reference is also made toabove-incorporated U.S. Pat. No. 5,769,880. However, it should beappreciated that the inventive concepts disclosed and described hereinare not limited to implementation in the NovaSure endometrial ablationsystem, and may also be employed in other tissue ablation systems.

Referring to FIGS. 1A and 1B, an intrauterine ablation device 100suitable for use with an RF generator 116 is comprised generally ofthree major components: an RF applicator head 103, a sheath 105, and ahandle 106. The applicator head 103 is slidably disposed within thesheath 105 (FIG. 1A) during insertion of the device into the uterinecavity, and the handle 106 is subsequently manipulated, as indicated byarrow A1, to cause the applicator head 103 to extend from the distal endof the sheath 105 (FIG. 1B). As seen in FIG. 1B, the applicator head 103extends from the distal end of a length of tubing 108, which is slidablydisposed within the sheath 105. The applicator 103 includes an externalelectrode array 103 a and an internal deflecting mechanism 103 b used toexpand the array for positioning into contact with the tissue.

The RF electrode array 103 a is formed of a stretchable metallizedfabric mesh, which is preferably knitted from a nylon and spandex knitplated with gold or other conductive material. Insulating regions 140(FIGS. 1C, 1D) are formed on the applicator head to divide the mesh intoelectrode regions. The insulated regions 140 may be formed using etchingtechniques to remove the conductive metal from the mesh, althoughalternate methods may also be used, such as by knitting conductive andnon-conductive materials together to form the array. The array may bedivided by the insulated regions 140 into a variety of electrodeconfigurations. In a preferred configuration shown in FIG. 1C, theinsulating regions 140 divide the applicator head into four electrodes142 a-142 d by creating two electrodes on each of the broad faces 134.To create this four-electrode pattern, the insulating regions 140 areplaced longitudinally along each of the broad faces 134, as well asalong the length of each of the faces 136, 138. The electrodes 142 a-142d are used for ablation and to measure tissue impedance during use.

The deflecting mechanism 103 b and its deployment structure are enclosedwithin the electrode array 103 a. As seen in FIG. 1B, an externalhypotube 109 extends from tubing 108, and an internal hypotube 110 isslidably and co-axially disposed within the external hypotube 109.Flexures 112 extend from the tubing 108 on opposite sides of thehypotube 109. A plurality of longitudinally spaced apertures (not shown)may be formed in each flexure 112. During use, these apertures allowmoisture to pass through the flexures, and to be drawn into exposeddistal end of hypotube 109 using a vacuum source 140 fluidly coupled tothe hypotube 109 at vacuum port 138. The internal flexures 116 extendlaterally and longitudinally from the exterior surface of hypotube 110,and are each connected to a corresponding one of the flexures 112. Atransverse ribbon 118 extends between the distal portions of theinternal flexures 116. The transverse ribbon 118 is preferablypre-shaped such that, when in the relaxed condition, the ribbon assumesthe corrugated configuration (shown in FIG. 1B), and such that, when ina compressed condition, the ribbon is folded along the plurality ofcreases 120 that extend along its length.

The deflecting mechanism 103 b formed by flexures 112, 116 and ribbon118 shapes the array 103 a into the substantially triangular shape(shown in FIG. 1B), which is particularly adaptable to most uterineshapes. During use, distal and proximal grips 144, 146 forming handle106 are squeezed towards one another to deploy the array. This actionresults in relative rearward motion of the hypotube 109, and relativeforward motion of the hypotube 110. The relative motion between thehypotubes causes deflection in flexures 112, 116, which thereby deploysand tensions the electrode array 103 a.

The flexures 112, 116 and ribbon 118 are preferably made from aninsulated spring material such as heat treated 17-7 PH stainless steel.Each flexure 112 preferably includes conductive regions that areelectrically coupled to the array for delivery of RF energy to the bodytissue. Strands of nylon thread 145 are preferably sewn through thearray 103, and around the flexures 112, in order to prevent theconductive regions 132 from slipping out of alignment with theelectrodes 142 a-142 d.

As mentioned above, the RF generator system in the existing NovaSureendometrial ablation system transmits RF ablation power based on thesurface area of the target ablation tissue. For uterine ablation, the RFpower is calculated using the measured length and width of the uterus.While these measurements may be made using conventional intrauterinemeasurement devices, the ablation device 100 of the NovaSure endometrialablation system has been customized to be used to measure the uterinewidth by transducing the separation of flexures using a mechanical orelectrical transducing means. In particular, referring again to FIG. 1B,the ablation device 100 includes non-conductive (e.g. nylon) suturingthreads 122 that extend between the hypotube 110 and the distal portionof the deflecting mechanism (FIG. 1B). Threads 122 are connected to anelongate wire (not shown) which extends through the tubing and iscoupled to a mechanical transducer such as a rotatable bobbin (notshown) or an electrical transducer such as a strain gauge electricallycoupled to an A/D converter to electrically transduce the separationdistance of the flexures 112 and to electronically transmit the uterinewidth to a visual display and/or directly to the RF generator.

Model and Scaled Model Tissue Ablation Impedance Curves

In accordance with a general aspect of the disclosed inventions, thesystem controller of an endometrial lining tissue ablation system, suchas the NovaSure endometrial ablation system, is programmed to monitorthe tissue impedance throughout the ablation process, and to modulate(i.e., increase, decrease, or maintain same) the delivered power atregular power adjustment intervals so that the measure tissue impedancetracks a “model” ablation procedure impedance curve, in order toachieved a complete and uniform ablation of the endometrial liningtissue in a standard procedure time, regardless of uterine cavity size,tissue moisture content, or other factors that traditionally varygreatly the procedure power level and duration.

In particular, the present inventors have derived a preferred or “model”target endometrial lining tissue ablation impedance curve, based onhistoric clinical data, for achieving a clinically complete ablation, asindicated by reaching a tissue impedance of approximately fifty ohms atthe end of an ablation procedure lasting approximately two minutes. Themodel impedance curve is 200 is depicted in FIG. 2, and represents apreferred or “target” endometrial lining tissue impedance (in ohms) 202as a function of time (t, in seconds) 204, assuming an initial tissueimpedance of (or about) five ohms. Notably, the initial tissue impedanceis measured after a relatively low level RF energy (55 watts) has beendelivered to the endometrial lining tissue for five seconds, in order toreduce the initial moisture content of the uterine cavity and stabilizethe tissue so that the measured impedance more accurately represents thetissue impedance rather than excess moisture (e.g., saline) present inthe uterine cavity.

In accordance with a more specific aspect of the disclosed inventions,and with reference to the table 300 shown in FIG. 3, the presentinventors have determined a mathematical formula that provides discreteimpedance values 302 at one second intervals 304 for the entire twominute ablation procedure, which impedance values approximate the modelcurve 200 shown in FIG. 2:

I _(T)=4+(49^((T/120)))  (1)

where I_(T) is a target tissue impedance at a given time T (in seconds)of the ablation time period.

By providing target impedance values at precise timing intervals for anentire procedure, the system controller output need only compare thetarget value (e.g., the values in 302 in table 300) with the actualmeasured value for each timing interval, and then adjust the deliveredpower accordingly. This process of adjusting the delivered power atregular timing intervals is referring to herein as modulating the outputpower, although it may be the case that the power level is notnecessarily changed at any given power adjustment interval.

Of course, the model procedure impedance curve 200 shown in FIG. 2assumes that the initial endometrial lining tissue impedance (i.e.,after five seconds of power delivery) will be five ohms; whereas theactual initial impedance may vary significantly from as little as underone ohms, or as much as twelve or more ohms, depending on uterine size(i.e., the amount of endometrial lining tissue to be ablated), tissuemoisture content, and other factors. While one approach would be todrive the tissue impedance up or down, as may be the case, as quickly aspossible to reach (and thereafter track) the model impedance curve 200,a more preferred approach is to track a scaled impedance curve thatgenerally follows the same trajectory as the model impedance curve 200and still ends up at approximately 50 ohms after approximately twominutes total procedure time, without risking an incomplete overover-done ablation in an effort to force the procedure to follow curve200.

By way of illustration, a scaled model impedance curve 400 is shown inFIG. 4, which is based upon an initial tissue impedance of (or about)ten ohms after 55 watts RF energy has been delivered to the endometriallining tissue for five seconds. As with the model impedance curve 200,the scaled model impedance curve 400 represents a target endometriallining tissue impedance (in ohms) as a function of time (in seconds) foran ablation procedure reaching approximately fifty ohms impedance inapproximately two minutes, but starting at ten ohms instead of fiveohms.

By way of further illustration, another scaled model impedance curve 600is shown in FIG. 6, which is based upon an initial tissue impedance of(or about) two ohms after 55 watts RF energy has been delivered to theendometrial lining tissue for five seconds. As with the model impedancecurves 200 and 400, the scaled model impedance curve 600 represents atarget endometrial lining tissue impedance (in ohms) as a function oftime (in seconds) for an ablation procedure reaching approximately fiftyohms impedance in approximately two minutes, but this time starting attwo ohms, instead of five or ten ohms. By way of still furtherillustration, FIG. 8 is a compilation of the tissue impedance curves200, 400, 600 of FIGS. 2, 4 and 6, wherein the respective curvesconverge at approximately fifty ohms at about the two minute mark.

It will be appreciated that, in a preferred implantation, the ablationsystem controller is preferably programmed using a single formula todetermine a set of target impedance values for an ablation procedurebased on a given measured initial impedance. Towards this end, and withreference to table 500 shown in FIG. 5, and table 700 shown in FIG. 7,the present inventors have arrived at a further mathematical formulathat takes into account the initial measured impedance value, andprovides a set of discrete, one second interval target impedance values,I_(t), that approximate a model impedance curve corresponding to therespective initial measured impedance, as follows:

I _(t) =I _(max)−((I _(max) −I _(o))*S)  (2)

where I_(t) is the target tissue impedance (in ohms) for a given time t(in seconds) following the initial power delivery time period,

I_(max) is a target maximum tissue impedance,

I_(o) is a tissue impedance at time t derived from the above formula (1)approximating the preferred endometrial lining tissue ablation impedancecurve 200, and

S is a scaling factor equal to (I_(max)−I_(meas))/(I_(max)−I_(io)),where I_(meas) is a measured impedance of the endometrial lining tissueafter RF power has been delivered for the initial time periodt_(initial), and I_(io) is a tissue impedance at time t_(initial)derived from the function approximating the preferred endometrial liningtissue ablation impedance curve.

Thus, the target impedance values 502 for the respective one secondintervals 304 shown in table 500 (FIG. 5) are derived from above formula(2) and approximate the model curve 400 (FIG. 4) for an initial measuredtissue impedance of ten ohms. Similarly, the target impedance values 702for the respective one second intervals 304 shown in table 700 (FIG. 7)are derived from above formula (2) to approximate the model curve 600(FIG. 6) for an initial measured tissue impedance of two ohms. As can beseen in FIG. 8, the target impedance for all curves 200, 400, 600converge at approximately fifty ohms at approximately 120 seconds intothe procedure.

It will be appreciated by those skilled in the art that the using theabove formula (2) to generate a table of discrete target impedancevalues for a given procedure based on the measured initial impedance isstraight forward to implement in a programmable ablation systemcontroller, such as the controller of a NovaSure endometrial ablationsystem or a comparable tissue ablation system.

Preferably, the delivered RF power is substantially constant for thetime period prior to the initial measured impedance from which thetarget impedance values are derived. However, it is not required to beconstant, so long as the initial time period and delivered RF powerlevel are together sufficient such that the initial measured impedanceis fairly representative of the endometrial lining tissue. In apresently preferred embodiment, constant power at fifty-five watts isdelivered for five seconds prior to obtaining the initial tissueimpedance measurement from which the set of target impedance values arederived for the procedure. Thereafter, the delivered power may bedecreased, increased, or maintained constant, as appropriate, at eachpower adjustment interval (discussed below in greater detail), with aminimum delivered power of 20 watts, and a maximum delivered power of180 watts. The controller may be programmed so that RF power isdelivered to the endometrial lining tissue until either (i) the measuredimpedance of the endometrial lining tissue reaches a predeterminedmaximum tissue impedance (e.g., fifty ohms), or (2) a maximum proceduretime is reached (e.g., two minutes), whichever occurs first.

Thus, in a preferred embodiment, an endometrial tissue ablation systemwill include an RF generator operatively coupled an RF applicatorconfigured for being positioned within a uterine cavity and forcontacting and ablating endometrial lining tissue of the uterine cavity,and a system controller operatively coupled with the RF generator,wherein the controller is configured to (a) cause the generator todeliver an RF current through the one or more applicator electrodes tothereby deliver a corresponding RF power to endometrial lining tissue ofa uterine cavity in which the applicator is positioned, and (b) modulatethe delivered RF current to thereby modulate the delivered RF power sothat a measured impedance of the endometrial lining tissue tracks atarget tissue impedance as a function of time, wherein the target tissueimpedance is derived from a function that approximates a preferredendometrial lining tissue ablation impedance curve that is based upon ameasured impedance of the endometrial lining tissue after RF power hasbeen delivered for a predetermined initial time period. The systemcontinues to deliver RF power to the endometrial lining tissue until theactual tissue impedance reaches a predetermined maximum, or a maximumprocedure time is reached, whichever occurs first.

Power Modulation Options

As discussed above, once the ablation system controller has measured theinitial endometrial lining tissue impedance and calculated a set ofdiscrete target tissue impedance values for the entire procedure, thecontroller will thereafter continue to measure the tissue impedance atregular power adjustment intervals, and modulate the delivered power(which corresponds directly to the RF output current), as needed, inorder to closely track the respective calculated target tissue impedancevalues.

The process of modulating output power based on measured tissueimpedance is well known in the art of RF ablation controllers. Inimplementations of the presently disclosed inventions, the poweradjustment intervals are preferably no longer than, and more preferablyrelatively short compared with, the one second target impedanceintervals. By way of non-limiting examples, the power adjustmentintervals may be each second, 500 milliseconds, 250 milliseconds, 100milliseconds, or of even less duration. Preferably, the delivered powerneed not be increased or decreased, as applicable, more than arelatively small percentage difference to maintain tracking of thetarget impedance value, which changes no more frequently than one secondintervals, to avoid possible over-compensation at each adjustment. Theabsolute amount of a given power adjustment at each interval may bedynamic (e.g., based on the difference between the measured and targetimpedance values), or static (e.g., a uniform percentage increase ordecrease, as applicable. Either way, it is preferred that no oneinterval adjustment be more than about 3% to about 5% of the currentpower level.

By way of example, in an exemplary embodiment, an endometrial liningtissue ablation procedure is commenced, and RF current is transmitted todeliver 55 watts power to the endometrial lining tissue for an initialfive seconds, at which time an initial tissue impedance measurement isobtained and the corresponding target impedance values for the remainingprocedure are calculated using above formula (2). 250 millisecondslater, i.e., 5.25 total seconds into the procedure, the systemcontroller again measures the actual tissue impedance, and compares thismeasured impedance with the calculated target impedance for t=6 seconds.If the measured tissue impedance is less than the target impedance fort=6 seconds, then the delivered power is increased by up to 3% (i.e., byup to 1.515 watts) to a new delivered power level of up to 56.515 watts.Conversely, if the measured tissue impedance is greater than the targetimpedance for t=6 seconds, then the delivered power is decreased by nomore than 1.515 watts, to a new delivered power level of no less than53.485 watts. And, of course, if the measured tissue impedance happensto be equal to the target impedance for t=6 seconds, the power level ismaintained constant. 250 milliseconds later, i.e., at 5.5 total secondsinto the procedure, the process is repeated, and again until the end ofthe procedure at approximately 120 total seconds. Thus, in thisembodiment, there are four power adjustment intervals per second, with amaximum possible increase or decrease of about 12% per second.

In various embodiments, the controller may be programmed to reduce thepercentage increase or decrease if the measured impedance for a givenpower adjustment interval is within close range to the target impedanceto avoid overcompensation. Further, in some embodiments the controllermay be programmed to not make any adjustments to (i.e., to maintainconstant) the current delivered power if the measured impedance iswithin a certain percentage (e.g., within 1% to 3%) of the targetimpedance at the end of a given interval. In a presently preferredembodiment, the delivered power is maintained within a range of not lessthan 20 watts and not greater than 180 watts.

Importantly, the total amount of energy delivered during an endometriallining tissue ablation using above-described energy is generallyoptimal, regardless of the uterus size, given the 120 second limit.

Although particular embodiments have been shown and described, it willbe understood that they are not intended to limit the presentinventions, and it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thescope of the present inventions. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense, and the present inventions are intended to cover alternatives,modifications, and equivalents thereof, which may be included within thescope of the present inventions as defined by the claims.

What is claimed is:
 1. A method for performing an endometrial liningtissue ablation procedure, the method comprising: positioning aradiofrequency (RF) applicator within a uterine cavity, wherein the RFapplicator is electrically coupled with an RF generator; delivering RFpower from the RF generator to endometrial lining tissue within theuterine cavity via the RF applicator; after delivering the RF power tothe endometrial lining tissue for an initial time period, measuring aninitial impedance of the endometrial lining tissue using an impedancemeasurement circuit provided by the RF applicator; based upon themeasured initial impedance, determining a target impedance of theendometrial lining tissue as a function of time for a duration of theendometrial lining ablation procedure; and modulating the RF powerdelivered to the endometrial lining tissue during the endometrial liningablation procedure so that an impedance of the endometrial lining tissuetracks the target impedance.
 2. The method of claim 1, wherein thetarget impedance of the endometrial lining tissue for the duration ofthe endometrial lining ablation procedure is based in part upon scalinga preexisting endometrial lining tissue ablation impedance curve basedupon the measured initial impedance of the endometrial lining tissue. 3.The method of claim 1, wherein the controller is configured to determinethe target impedance of the endometrial lining tissue for the durationof the endometrial lining ablation procedure based in part uponimpedance data obtained from prior performed endometrial lining tissueablation procedures.
 4. The method of claim 1, wherein the RF applicatorcomprises an electrode carrier including one or more bipolar electrodepairs thereon.
 5. The method of claim 1, wherein the RF power isdelivered to the endometrial lining tissue at a substantially constantpower level during the initial time period.
 6. The method of claim 5,wherein a duration of the initial time period and the substantiallyconstant power level of the delivered RF power during the initial timeperiod are together sufficient such that the measured initial impedanceis representative of an electrical conductivity of the endometriallining tissue.
 7. The method of claim 2, wherein the target impedance ofthe endometrial lining tissue for the duration of the endometrial liningablation procedure is determined according to a formula,I_(t)=I_(max)−((I_(max)−I_(o))*S), where I_(t) is a target tissueimpedance for a given time t during the endometrial lining ablationprocedure, I_(max) is a target maximum impedance of the endometriallining tissue, I_(o) is a projected impedance of the endometrial liningtissue for the given time t during the endometrial lining ablationprocedure derived from a function approximating the preexistingendometrial lining tissue ablation impedance curve, and S is a scalingfactor equal to (I_(max)−I_(meas))/(I_(max)−I_(io)), where I_(meas) isthe measured initial impedance of the endometrial lining tissue,t_(initial), and I_(io) is a projected initial impedance of theendometrial lining tissue derived from the function approximating thepreexisting endometrial lining tissue ablation impedance curve.
 8. Themethod of claim 7, wherein the duration of the endometrial liningablation procedure is about 120 seconds, I_(max) is about 50 ohms, andthe function approximating the preexisting endometrial lining tissueablation impedance curve is I_(t)=4+(49^((T/120))), where T is a giventime (in seconds) during the endometrial lining ablation procedure. 9.The method of claim 1, wherein the RF power is delivered to theendometrial lining tissue until (i) the impedance of the endometriallining tissue reaches a target maximum tissue impedance, or (ii) aduration of the endometrial lining ablation procedure reaches a targetcompletion time, whichever occurs first.
 10. The method of claim 1,wherein the delivery of the RF power to the endometrial lining tissue ismodulated by causing one of increasing, decreasing, or maintainingconstant a delivered RF current to the endometrial lining tissue atregular power adjustment intervals.
 11. The method of claim 10, wherein,at each power adjustment interval, an amperage of the delivered RFcurrent is not increased or decreased more than a predeterminedadjustment limit.
 12. The method of claim 11, wherein the predeterminedadjustment limit is about 3%.
 13. A method for performing an endometriallining tissue ablation procedure, the method comprising: positioning aradiofrequency (RF) applicator within a uterine cavity, wherein the RFapplicator is electrically coupled with an RF generator; delivering RFpower from the RF generator to endometrial lining tissue within theuterine cavity via the RF applicator; after delivering the RF power tothe endometrial lining tissue for an initial time period, measuring aninitial impedance of the endometrial lining tissue using an impedancemeasurement circuit provided by the RF applicator; modulating the RFpower delivered to the endometrial lining tissue during the endometriallining ablation procedure so that an impedance of the endometrial liningtissue tracks a target impedance of the endometrial lining tissue as afunction of time for a duration of an endometrial lining ablationprocedure derived from impedance data obtained from prior performedendometrial lining tissue ablation procedures; and stopping delivery ofthe RF power to the endometrial lining tissue when (i) the impedance ofthe endometrial lining tissue reaches a target maximum tissue impedance,or (ii) the duration of the endometrial lining ablation procedurereaches a target completion time, whichever occurs first.
 14. The methodof claim 13, wherein the delivery of the RF power to the endometriallining tissue is modulated by causing one of increasing, decreasing, ormaintaining constant a delivered RF current to the endometrial liningtissue at regular power adjustment intervals.
 15. The method of claim13, wherein, at each power adjustment interval, an amperage of thedelivered RF current is not increased or decreased more than apredetermined adjustment limit.
 16. The system of claim 13, wherein thetarget impedance of the endometrial lining tissue for the duration ofthe endometrial lining ablation procedure is determined according to aformula, I_(t)=I_(max)−((I_(max)−I_(o))*S), where I_(t) is a targettissue impedance for a given time t during the endometrial liningablation procedure, I_(max) is a target maximum impedance of theendometrial lining tissue, I_(o) is a projected impedance of theendometrial lining tissue for the given time t during the endometriallining ablation procedure derived from a function approximating apreexisting endometrial lining tissue ablation impedance curve, and S isa scaling factor equal to (I_(max)−I_(meas))/(I_(max)−I_(io)), whereI_(meas) is the measured initial impedance of the endometrial liningtissue, and I_(io) is a projected initial impedance of the endometriallining tissue derived from the function approximating the preexistingendometrial lining tissue ablation impedance curve.
 17. The system ofclaim 16, wherein the duration of the endometrial lining ablationprocedure is about 120 seconds, I_(max) is about 50 ohms, and thefunction approximating the preexisting endometrial lining tissueablation impedance curve is I_(t)=4+(49^((T/120))), where T is a giventime (in seconds) during the endometrial lining ablation procedure.